The invention relates to electronic devices, and more particularly to dispersive optical systems, compensation methods, optical transfer function designs, and corresponding devices.
The performance of long-haul and high-speed dense wavelength division multiplexed (DWDM) optical communication networks depends upon monitoring and adapting to changing circumstances such as load variations, signal degradation, dispersion, and so forth. Indeed, within a single-mode optical fiber the range of (free space) wavelengths from roughly 1540 nm to 1570 nm (the xe2x80x9cC-bandxe2x80x9d) may be partitioned into channels with each channel including a width 0.2 nm of used wavelengths and adjacent 0.2 nm of unused wavelengths (e.g., 50 GHz periodicity); and links of such optical fiber may have lengths of several thousand km. Similarly for the L-band (roughly 1580 nm to 1610 nm). Data pulses formed from frequencies confined to the wavelengths of a single channel initially do not interfere with pulses formed from frequencies of another channel assigned to a different wavelength, and thus multiple data pulses from different sources may simultaneously propagate down the fiber. Clearly, narrow channel spacing provides greater overall data rates, but requires greater limits on non-linearities of the optical fibers and attendant devices. Indeed, optical networks have various problems such as the following.
(1) Non-uniform gain across wavelengths by the typical erbium doped fiber amplifier (EDFA) leads to signal power non-linearities and cross-talk between channels, and static gain flattening cannot adapt to changing circumstances.
(2) Large power transients arising from adding/dropping of channels and optical switching with cascaded EDFAs.
(3) Optical filters typically have static characteristics and cannot track dynamic changes to EDFAs and add/drop channels.
(4) Multi-band dispersion and chromatic dispersion create channel interference over long-hauls and limit transmission length to roughly 1/BDxcex94xcex where B is the bit rate (pulse repetition rate), D is the dispersion in ps/nm/km, and xcex94xcex is the channel bandwidth. Various approaches to compensation for this dispersion include chirped fiber Bragg gratings together with optical circulators which cause differing wavelengths to travel differing distances to compensate for the dispersion. Also, U.S. Pat. No. 6,310,993 discloses chromatic dispersion compensation by use of a virtually imaged phased array.
Digital micromirror devices (DMD) provide a planar array of micromirrors (also known as pixels) with each micromirror individually switchable between an ON-state and an OFF-state in which input light reflected from a micromirror in the ON-state is directed in one direction and the light reflected from an OFF-state micromirror is directed in another direction. DMD arrays may have sizes on the order of 1000xc3x971000 micromirrors with each micromirror on the order of 10 umxc3x9710 um. See U.S. Pat. No. 6,323,982 for a recent DMD description.
The present invention provides optical filtering according to both amplitude and phase by a transfer function approach. Such optical filtering provides dispersion compensation in an optical network or optical system. Preferred embodiment optical filters include DMD devices with wavelengths dispersed over the array of micromirrors for individual attenuation.
This has advantages including increased capacity for digital optical wavelength division systems.